Multiple Personalities - Caused by Writing

Can't say I've ever experienced that, but I'm sure if you spend a lot of time with one character, developing them and understanding them you can certainly feel like you know them really well and become attached to them. I guess for that reason it wouldn't be surprising if they end up affecting the way you behave, a bit like the way that method actors like Daniel Day-Lewis work.

I suppose it puts a new spin on the term 'brain-child'

But yeah, I know what you mean, sort of. I think we as writers create worlds and then populate them inside our minds, a downside is that a really good and deep world might draw us in too deeply. Just be careful that the line between reality and fantasy stays firm.

I don't think so. You really can only get MPD, if you've always had it in the first place. I've noticed that while falling in love with tv characters like Fan Bing Bing's "Yuhe", or Eri Fukatsu's, "Isaki-san," I do pick up some little quirks and dressing style. But then again, I pick up little quirks I like from my boyfriend as well.
I honestly think it's a natural tendency in people.

Some just take it a little farther than most, like - say - cosplaying their favorite characters at conventions.

I'm convinced that all writers are just frustrated actors (and all actors are all frustrated somethingelse's) and it would only be natural that some take on the rolls more deeply than others. Maybe you're a method writer-the Daniel Day Lewis of the forum...

As an actor who has come to writing later, I feel Mr. Travis is definitely on the right track in some respects. Have experienced an immersion, if you will, with certain characters that I've created and by using the feelings generated, the dialogue and action almost drives itself. Not to go all Zen/California/Naval-Gazing on you, but it's definitely happened and I pray it will continue to happen.

PS: When the Bard said,
"All the world's a stage,
And all the men and women merely players:
They have their exits and their entrances;
And one man in his time plays many parts."
he weren't just a-whistlin' Dixie!